Cold start auxiliary circuit for electronic fuel control system

ABSTRACT

An auxiliary circuit means is disclosed herein for controlling actuation of an injector valve means to provide for cold starting of an engine. The circuit and injector means are energized when a vehicle ignition system is energized and the fuel for cold starting is provided during the period of time required for a selected voltage value to raise to a threshold value. The threshold value is determined by a voltage divider including a temperature-responsive element sensing engine temperature. Means are provided to render the auxiliary circuit insensitive to supply voltage variations.

United States Patent Rachel Mar. 7, 1972 [54] COLD START AUXILIARYCIRCUIT 3,504,657 4/1970 Eichler et al. ..123/119 R X FOR LE TR NI FUELCONTROL 3,533,381 10/1970 Schmid... ..123/32 EA SYSTEM 3,513,815 5/1970Mair ..l23/32 EA [72] lnventor: Todd L. Rachel, Elmira, NY.

[73] Assignee: The Bendix Corporation [22] Filed: June 16, 1970 [21]Appl. No.: 46,706

[52] US. Cl. ..l23/32 EA, 123/139 E, 123/179 G, 123/ 187.5 R

[51] Int. Cl ..F02m 51/00 [58] Field oiSearch.... ..123/32 EA, 119,179 A[56] References Cited UNITED STATES PATENTS 2,807,244 9/1957 Barclay..123/32 EA T0 COLD START C/RC'U/T Primary Examiner-Laurence M.Goodridge Attorney-Robert A. Benziger and Plante, Hartz, Smith andThompson [57] ABSTRACT An auxiliary circuit means is disclosed hereinfor controlling actuation of an injector valve means to provide for coldstarting of an engine. The circuit and injector means are energized whena vehicle ignition system is energized and the fuel for cold starting isprovided during the period of time required for a selected voltage valueto raise to a threshold value. The threshold value is determined by avoltage divider including a temperature-responsive element sensingengine temperature. Means are provided to render the auxiliary circuitinsensitive to supply voltage variations.

9 Claims, 3 Drawing Figures PAIENTEDMAR 7 m2 SHEET 1 UF 2 To COLD STARTWITNESS: 5%. m

ATTORNEY PATENTEBHAR 1 m2 3,646,915

' SHEET 2 UF 2 12$ gioa 76 70 73 g l 4% A 10 7 57-3 I I INVEN'IUR.

QMA

WITNESS B ATTORZVE Y COLD START AUXILIARY CIRCUIT FOR ELECTRONIC FUELCONTROL SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS The present caseis related to copending, commonly assigned application Ser. No. 46,681,Cold Start Auxiliary Circuit for Electronic Fuel Control System, by JohnR. Nagy et al., filed on June 16, 1970, and Ser. No. 46,705, AuxiliaryCircuit for Electronic Fuel Control System to Facilitate Cold Starting,by John R. Nagy, Filed on June 16, 1970.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to improvements in electronic fuel control systems andparticularly to improvements in automotive electronic fuel controlsystems whereby the cold start function is provided. In particular, thepresent invention provides a cold start circuit which is more reliablethan previous circuits in that it is resistant to variations in thesupply voltage and to false triggering.

2. Description of the Prior Art The known electronic fuel controlsystems currently rely upon the input information from their variousparameter sensors to provide the information required by an electronicfuel control main computing system to provide cold start enrichment.These sensors, generally, sense the engine temperature which may be thetemperature of the water jacket, to indicate the operating temperatureof the engine, the engine speed to determine timing and engine fuelrequirements, the intake manifold pressure to sense the load on theengine, and various other parameters as needed or desired.

For the purpose of this specification a cold" engine is one which, inattempting to assume ambient air temperature, has

cooled to a temperature below a selected level. This selected level maybe empirically determined and is the temperature below which thedifficulty of starting is increased beyond the capability of the maincomputing system to handle efficiently. The present electronic fuelcontrol systems rely upon the engine temperature sensor input (or anequivalent such as ambient air temperature and cylinder headtemperature) to vary the duration of injector valve open timesufficiently to provide fuel for the startup of the engine when theengine is cold. However, investigation has shown that this means ofproviding sufficient fuel for cold starting of an associated engine isnot always adequate. It is, therefore, an objective of the presentinvention to provide control circuitry in addition to the electronicfuel control system to control provision of sufficient quantities offuel for cold starting of an associated engine. It is a further objectof the present invention to provide control circuitry for an electronicfuel control system which is capable of providing sufficient fuel forstarting of an associated engine over a broad range of environmentaltemperatures.

One of the principal difficulties with the presently proposed methods ofproviding a cold-starting charge of fuel is that failure of the engineto start will permit excess quantities of fuel to be injected, restingin engine flooding. This problem also occurs upon too-frequent actuationof the cold start injector valve means. It is therefore an object of thepresent invention to provide a cold start auxiliary circuit which willnot cause a flooding problem. More particularly it is a further objectof the present invention to provide such a circuit which is adapted toenergize an injector valve means a predetermined number of times foreach energization of the starting motor. It is a still further object ofthe present invention to provide control circuitry to control theinjection of a quantity of fuel sufficient to permit starting of a coldengine, which quantity of fuel may vary as a function of the temperaturedrop below a predetermined threshold value but which is independent ofengine cranking. It is also an object of the present invention toprovide a cold start circuit which prevents too-frequent actuation ofthe cold start injector valve means.

It is believed that many of the difiiculties encountered by the presentmethod of cold starting are caused by the low r.p.m. of the associatedengine during the cranking cycle and the proximity of the main injectornozzles to the intake ports of the engine to be started. It is,therefore, an object of the present invention to provide a circuit, inaddition to the main electronic fuel control computing circuit,responsive to temperature for controlling an injector nozzle or valvemeans which may be situated independently of the main injector nozzles.It is a still further object of the present invention to provide a coldstart fuel controlling circuit which may control the provision of acharge of fuel to the engine to be started independent of the enginecranking speed.

In the commonly assigned copending application MOC 69/ 18-3, a circuitfor providing the desired colt start function is shown and described.However, the embodiment described therein, which includes twosuccessively triggered monostable multivibrators, is known to besensitive to variations in the level of the B+ voltage and tosusceptible to premature, or false, triggering. These difficulties ariseout of the use of successively triggered multivibrators so that itbecomes a still further object of the present invention to provide acold start circuit having the above-described advantages which does notemploy multivibrators. It is an object of the present invention toprovide a cold start auxiliary circuit whose output signal is relativelyinsensitive to variation of the supply over a wide range of voltages. Itis still a further object of the present invention to provide a coldstart auxiliary circuit which is insensitive to false triggeringsignals. In order to accomplish the last enumerated objective, it is anobject of the present invention to provide a cold start auxiliarycircuit for electronic fuel injection in which a substantial period ofoff", or inactive, time is required to enable the circuit to bereenergized.

SUMMARY OF THE PRESENT INVENTION The present invention provides aspecial function (cold start) auxiliary circuit for an electronic fuelcontrol system capable of providing the improved cold start function.The cold start auxiliary circuit is adapted to provide an output signalof variable duration with the duration being a function of the drop ofengine temperature below a selected level. The signal can be appliedto-an injector nozzle or valve which may be situated independently fromthe main injector nozzles of the associated engine. The circuit isfurther adapted to be selfdisabling after generating a single pulse foreach energization of the starting circuit of the associated engine tothereby prevent flooding of the engine. The inventive circuit ischaracterized by providing an output signal whose duration issubstantially independent of the supply voltage. The circuit is furthercharacterized by the fact that the circuit can be activated to energizethe associated injector means only once for each application of supplyvoltage, after which time the supply voltage must be removed for aperiod of time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows, in diagrammatic circuitform, an electronic fuel control system main computation circuit asadapted, for instance, for automotive use.

FIG. 2 shows, in diagrammatic circuit form, an auxiliary circuitaccording to the present invention for providing the cold startfunction.

FIG. 3 shows the relationship of the present invention to an automotiveelectrical system.

DETAILED DESCRIPTION Referring now to FIG. 1, an electronic fuel controlsystem main computation circuit 10 is shown. The circuit is shown asbeing energized by a voltage supply designated as B+ at the variouslocations noted. In the application of this system to an automotiveengine fuel control system, the voltage supply could be the batteryand/or battery charging system conventionally used as the vehicleselectric power source. The man skilled in the art will recognize thatthe electrical polarity of the voltage supply could readily be reversed.

The circuit receives, along with the voltage supply, various sensoryinputs, in the form of voltage signals in this instance, indicative ofvarious operating parameters of the associated engine. Intake manifoldpressure sensor 12 supplies a voltage indicative of manifold pressure,temperature sensor 14 is operative to vary the voltage across theparallel resistance associated therewith to provide a voltage signalindicative of engine temperature and voltage signals indicative ofengine speed are received at circuit input port 16. This signal may bederived from any source indicative of engine crank angle but ispreferably from the engines ignition distributor, not shown.

The circuit 10 is operative to provide two consecutive pulses, ofvariable duration, through sequential networks to circuit location 18 tothereby control the on time of transistor 20. The first pulse isprovided via resistor 22 from that portion of circuit 10 having inputsindicative of engine crank angle and intake manifold pressure. Thetermination of this pulse initiates a second pulse which is provided viaresistor 24 from that portion of the circuit 10 having aninput from thetemperature sensor 14. These pulses, received sequentially at circuitlocation 18, served to turn transistor on (that is, transistor 20 istriggered into the conduction state) and a relatively low voltage signalis present at circuit output port 26. This port may be connected,through suitable inverters and/or amplifiers (not shown) to the injectormeans (shown as 78 in FIG. 3) such that the selected injector means areenergized whenever the transistor 20 is on. It is the current practiceto use switching means to control which of the injector valve means arecoupled to circuit location 26 when the system is used for actuation ofless than all injector valve means at any one time. Because the injectorvalve means are relatively slow acting, compared with the speed ofelectronic devices, the successive pulses at circuit point 18 willresult in the injector valve means remaining open until after thetermination of the second pulse.

The duration of the first pulse is controlled by the monostablemultivibrator network associated with transistors 28 and 30. Thepresence of a pulse received via input port 16 will trigger themultivibrator into its unstable state with transistor 28 in theconducting state and transistor 30 blocked (or in the nonconductingstate). The period of time during which transistor 28 is conducting willbe controlled by the voltage signal from manifold pressure sensor 12.Conduction of transistor 28 will cause the collector 28C thereof toassume a relatively low voltage close to the ground or common voltage.This low voltage will cause the base of 34b of transistor 34 to assume alow voltage below that required for transistor 34 to be triggered intothe conduction state, thus causing transistor 34 to be turned off. Thevoltage at the collector 34c will, therefore, rise toward the B+ valueand will be communicated via resistor 22 to circuit location 18 where itwill trigger transistor 20 into the on" or conduction state thusimposing a relatively low voltage at circuit port 26. As hereinbeforestated, the presence of a low voltage signal at circuit port 26 willcause the selected injector valve means to open. When the voltage fromthe manifold pressure sensor 12 has decayed to the value necessary forthe multivibrator to relax or return to its stable condition, transistor30 will be triggered on and transistor 28 will be turned off. This will,in turn, cause transistor 34 to turn on, transistor 20 to turn off andthereby remove the injector control signal from circuit port 26.

During the period of time that transistor 34 has been held in thenonconducting, or off state, the relatively high voltage at collector34c has been applied to the base of transistor 36, triggering thetransistor 36 on". The resistor network 38, connected to the voltagesupply, acts with transistor 36 as a current source and current flowsthrough the conducting transistor 36 and begins to charge capacitor 40.Simultaneously, transistor 42 has been biased on and, with the resistornetwork 44, constitutes a second current source. Currents from bothsources flow into the base of transistor 46 thereby holding thistransistor on which results in a low voltage at the collector 460. Thislow voltage is communicated to the base of transistor 20 via resistor24.

When transistor 28 turns off" signalling termination of the first pulse,transistor 34 turns on and the potential at the col lector 34c falls toa low value. The current from the current source, comprised oftransistor 36 and resistor network 38, now flows through the base oftransistor 36 and the capacitor 40 ceases to charge. The capacitor willthen have been charged, with the polarity shown in FIG. 1, to a valuerepresentative of the duration of the first pulse. However, at the endof the pulse when transistor 34 is turned on, the collector-basejunction of transistor 36 is forward biased, thus making the positiveside of capacitor 40 only slightly positive with respect to ground sinceseveral PN junctions separate it from ground. This will impose anegative voltage on circuit location 48 which will reverse bias diode 50and transistor 46 will be turned off. This will initiate a high voltagesignal from the collector of transistor 46 to circuit location 18 viaresistor 24 which signal will retrigger transistor 20 on" and a secondinjector means control pulse will appear at circuit port 26. The timeduration between first and second pulses will be sufficiently short sothat the injector means will not respond to the brief lack of signal.

While the diode 50 is reverse biased, the current from the currentsource comprised of transistor 42 and resistor network 44 will beflowing through circuit location 48 and into the capacitor 40 to chargethe capacitor to the point that circuit location 48 will again bepositive. This will then forward bias diode 50 and transistor 46 willturn back on. This will ter minate the second pulse and the injectorvalve means, not shown, will subsequently close.

The duration of the second pulse will be a function of the time requiredfor circuit location 48 to become sufficiently positive for diode 50 tobe forward biased. This in turn is a function of the charge on capacitor40 and the magnitude of the charging current supplied by the currentsource comprised of transistor .42 and resistor network 44. The chargeon capacitor 40 is, of course, a function of the duration of the firstpulse. However, the rate of charge (i.e., magnitude of the chargingcurrent) is a function of the base voltage at transistor 42. This valueis controlled by the voltage divider networks 52 and 54 with the effectof network 54 being variably controlled by the engine temperature sensor14.

Referring now to FIGS. 1 and 2, and particularly to FIG. 2, a circuit isshown for providing the desired colt start characteristic. The circuit100 is also energized by B+ as noted. Circuit 100 receives a temperatureinput at circuit location A from the correspondingly designated portionof control circuit 10. Alphabetic designations are used herein to denotepoints common to circuits in several figures. The temperature input orsignal is comprised of a variable voltage whose value is controlled bythe engine temperature sensor 14, shown as a thermistor in FIG. 1. Thecold start circuit 100 is adapted to provide a single injection controlpulse at circuit location 102 to control energization of the cold startinjector valve means, shown as 76 in FIG. 3, which is preferablyphysically remote from the main electromechanical valve means. The coldstart auxiliary circuit 100 includes zener diode 104, an emitter coupledpair of transistors 106, 108 and transistor switches and 112. Thecircuit also includes various resistor, capacitor and diode combinationsto provide the desired voltage and current levels.

When power is applied at B+, as for instance by turning on of theignition switch the base 106b of transistor 106 will be at the groundpotential. The base 108!) of transistor 108 will be at some positivevoltage level due to the temperature indication signal applied at pointA due to the thermistor network shown in FIG. 1. This will causetransistor 108 to be conducting while transistor 106 is in thenonconducting state. The conduction of transistor 108 will cause acurrent flow through resistor 114 which will cause a voltage drop toappear across the emitter-base junction of transistor switch 110 whichwill cause transistor 110 to conduct. Conduction of transistor 110 willapply a voltage to the base 1l2b of transistor switch 112 which will, inturn, cause transistor 112 to conduct, thereby applying a voltage signalto circuit location 102 for application to the cold start injectormeans. During this time period the 13+ voltage via resistors 116 and 118has been charging capacitor 120, thereby causing the voltage at base106k of transistor 106 to increase. When the voltage at base l06bexceeds the voltage at base 108b, transistor 106 will turn on andtransistor 108 will turn off. The turning off of transistor 108 willcause transistor switches 110 and 112 to switch ofi, thereby terminatingcold start injection. The magnitude of the voltage signal applied atcircuit point A is related to the temperature drop below. a selectedlevel such that an engine temperature of 20 F. will produce asubstantially greater magnitude signal than will an engine temperatureof 50 F. By

proper tailoring of the rate at which the voltage at the base.

106b of transistor 106 will increase, the cold start injection periodmay be suitably tailored for various engines and various operatingconditions.

Zener diode 104 is operative to maintain the voltage across resistor 118and capacitor 120 at a fixed value regardless of the magnitude of B+.This, therefore, provides a stability of operation over a wide range ofoperating voltage levels such that the charging rate of capacitor 120 ismaintained uniform for all values of B+. In addition, in extremesituations where B+ drops below the zener diode threshold level,capacitor-120 will charge more slowly. This is of advantage inlengthening cold start injection in those extreme situations where anelectromechanical injector valve is energized by 8+ and is sluggish dueto the low instantaneous value of B+. This configuration is ofadditional advantage in that once capacitor 120 is charged to thethermistor network threshold value, the transistor switches aremaintained off while capacitor 120 continues to charge. Since capacitor120 is provided with a discharge path via resistors 118 and 122 and thethermistor network (of FIG. 1), switching off of the supply voltage willpermit the capacitor to discharge at a rate which, depending on thevalue of the capacitor and the various resistive values, will besufiiciently slow to prevent false, or premature, triggering of the coldstart injector valve means.

Referring'now to FIG. 3, the relationship of my invention to anautomotive electrical system is illustrated. The computing circuit 10andthe cold-starting auxiliary circuit 40 are shown as being connectedto a vehicle battery 70 through switch 72 which may be, for instance,the vehicle ignition switch. In addition, fuel pumping means 74 is alsoshown as being connected to the electrical system such that closure ofswitch 72 will energize computing circuit 10, cold-starting circuit 100and fuel pumping means 74. Circuits 10 and 100 are shown connected tofuel injector means 76 and 78 and control the energization thereof. Aswill be apparent from a consideration of FIGS. 2 and 3, once theinjector valve means 76 has been energized and turned off, the coldstart circuit will be in a charged up configuration and injector valvemeans 76 will not be reenergizable until after the circuit 100 has beendeenergized for a period of time, as for instance by opening switch 72.

The cold start injection auxiliary circuit accomplishes its statedobjectives. An output pulse is generated at circuit location 102 forapplication to the cold start injector valve means, not shown. The pulseduration is a function of the temperature drop of the associated enginebelow a selected level, independent of the supply voltage. Furthermore,the circuit is insensitive to false triggering since the circuit must bewithout power from the voltage supply (B+) for a period of timesufiicient to permit stored energy to be drained off before the coldstart injector valve control switches 110 and 112 may be switched backon.

lclaim:

1. In combination with a battery-energizable internal combustion enginefuel control system, wherein battery energization is variable, of thetype having engine-operating parameter sensor means including enginetemperature sensor means operative to generate a signal having a levelwhich increases in response to decreased in sensed engine temperature, acomputing means responsive to the engine sensor means for controllingthe actuation of injector valve means where the quantity of fueldelivered to the engine is controlled, the improvement comprisingcircuit means responsive to the engine temperature sensor operative toproduce an output pulse having a duration related to the level of sensedsignal whereby injector valve means may be energized by the pulse toprovide a quantity of fuel in proportion to pulse duration for coldstarting of the engine, said circuit including inhibiting meansoperative to limit said circuit means output pulse production to apredetermined maximum number of pulses per fuel system energization andfurther including energization level establishing means operative tomaintain energization of said circuit means at a preselected level forbattery variations above the preselected level and to variably energizesaid circuit means in proportion to battery variations below thepreselected level.

2. The system as claimed in claim 1 wherein said predetermined maximumnumber of pulses is one.

3. The system as claimed in claim 1 including further time delay meansoperative to maintain the circuit means in an inhibited state for apredeterminable period of time following deenergization of said circuitmeans.

4. The system as claimed in claim 3 wherein said time delay meanscomprise a chargeable electrical element and a discharge path having avalue which permits only a slow discharge of said element.

5. The system as claimed in claim 1 wherein said circuit meanscomprises:

an emitter coupled pair of transistors having a pair of bases,

one of said bases receiving the variable level signal indicative ofsensed engine temperature;

capacitive means for applying a second variable level signal to theother of said bases, said second signal having a level which varies withtime in a predeterrninable manner;

said transistors arranged for mutually exclusive conduction;

switching means responsive to the conductive state of one of saidtransistors to control actuation of the injector valve means; and

voltage regulator means for controlling the maximum voltage applied tosaid capacitive means. 6. The system as claimed in claim 5 wherein saidcapacitive means include a capacitor chargeable to a value in excess ofsaid variable level engine temperature signal and resistive means forproviding a slow discharge path for said capacitor whereby said othertransistor base will be suitably biased for conduction for a period oftime following removal of energization to prevent too frequent actuationof the associated injector valve means.

7. A fuel control system for engines having energizing means comprisingin combination:

sensory means operative to sense at least one operating parameter of theengine, including engine temperature sensor means operative to generatea signal having a level which varies in relation to sensed enginetemperature;

first energizable circuit means responsive to said sensory meansoperative to generate an output signal, said signal being comprised of aplurality of pulses having a duration indicative of the engine fuelrequirement;

injection valve means; at least a portion of said injection valve meansadapted to receive said output signal and to be periodicallyintermittently energized in response thereto to control fuel delivery tothe engine;

second energizable circuit means, including means responsive to enginetemperature, operative to generate a variable duration control pulse forapplication to at least a portion of said injection valve means uponenergization of said second circuit means when sensed engine temperatureis below a preselected level; and

inhibitory means coupled to said second circuit means operative toprevent reenergization of said second circuit means until said secondcircuit has been deenergized for a predeterminable period of time.

tive of engine temperature and a second signal which varies as afunction of time from second circuit energization; I

first switching means responsive to said comparison means having aconducting state and a nonconducting state and operative to be in one ofsaid states when said first signal is larger than said second signal andto be in the other of said states when said second signal is larger thansaid first signal; and

power follower switch means responsive to said first switch operative tochange state with said first switch and to thereby generate a poweroutput signal.

1. In combination with a battery-energizable internal combustion enginefuel control system, wherein battery energization is variable, of thetype having engine-operating parameter sensor means including enginetemperature sensor means operative to generate a signal having a levelwhich increases in response to decreased in sensed engine temperature, acomputing means responsive to the engine sensor means for controllingthe actuation of injector valve means where the quantity of fueldelivered to the engine is controlled, the improvement comprisingcircuit means responsive to the engine temperature sensor operative toproduce an output pulse having a duration related to the level of sensedsignal whereby injector valve means may be energized by the pulse toprovide a quantity of fuel in proportion to pulse duration for coldstarting of the engine, said circuit including inhibiting meansoperative to limit said circuit means output pulse production to apredetermined maximum number of pulses per fuel system energization andfurther including energization level establishing means operative tomaintain energization of said circuit means at a preselected level forbattery variations above the preselected level and to variably energizesaid circuit means in proportion to battery variations below thepreselected level.
 2. The system as claimed in claim 1 wherein saidpredetermined maximum number of pulses is one.
 3. The system as claimedin claim 1 including further time dElay means operative to maintain thecircuit means in an inhibited state for a predeterminable period of timefollowing deenergization of said circuit means.
 4. The system as claimedin claim 3 wherein said time delay means comprise a chargeableelectrical element and a discharge path having a value which permitsonly a slow discharge of said element.
 5. The system as claimed in claim1 wherein said circuit means comprises: an emitter coupled pair oftransistors having a pair of bases, one of said bases receiving thevariable level signal indicative of sensed engine temperature;capacitive means for applying a second variable level signal to theother of said bases, said second signal having a level which varies withtime in a predeterminable manner; said transistors arranged for mutuallyexclusive conduction; switching means responsive to the conductive stateof one of said transistors to control actuation of the injector valvemeans; and voltage regulator means for controlling the maximum voltageapplied to said capacitive means.
 6. The system as claimed in claim 5wherein said capacitive means include a capacitor chargeable to a valuein excess of said variable level engine temperature signal and resistivemeans for providing a slow discharge path for said capacitor wherebysaid other transistor base will be suitably biased for conduction for aperiod of time following removal of energization to prevent too frequentactuation of the associated injector valve means.
 7. A fuel controlsystem for engines having energizing means comprising in combination:sensory means operative to sense at least one operating parameter of theengine, including engine temperature sensor means operative to generatea signal having a level which varies in relation to sensed enginetemperature; first energizable circuit means responsive to said sensorymeans operative to generate an output signal, said signal beingcomprised of a plurality of pulses having a duration indicative of theengine fuel requirement; injection valve means; at least a portion ofsaid injection valve means adapted to receive said output signal and tobe periodically intermittently energized in response thereto to controlfuel delivery to the engine; second energizable circuit means, includingmeans responsive to engine temperature, operative to generate a variableduration control pulse for application to at least a portion of saidinjection valve means upon energization of said second circuit meanswhen sensed engine temperature is below a preselected level; andinhibitory means coupled to said second circuit means operative toprevent reenergization of said second circuit means until said secondcircuit has been deenergized for a predeterminable period of time. 8.The system as claimed in claim 7 wherein the energizing means exhibits avariable energization characteristic and including further: regulatingmeans to apply a maximum energization to said second circuit means, saidmaximum being less than the maximum energization available from theenergizing means; said-regulating means arranged to apply theinstantaneous level of energizing means energization when the level ofenergization drops below a preselected value.
 9. The system as claimedin claim 7 wherein said second circuit comprises: comparison meansadapted to receive a first signal indicative of engine temperature and asecond signal which varies as a function of time from second circuitenergization; first switching means responsive to said comparison meanshaving a conducting state and a nonconducting state and operative to bein one of said states when said first signal is larger than said secondsignal and to be in the other of said states when said second signal islarger than said first signal; and power follower switch meansresponsive to said first switch operative to change state with saidfirst switch and to thereby generate a power output signal.